Irritable bowel syndrome (IBS) is the most common of all gastrointestinal disorders, affecting 10-20% of the general population and accounting for more than 50% of all patients with digestive complaints. However, studies suggest that only about 10% to 50% of those afflicted with IBS actually seek medical attention. Patients with IBS present with disparate symptoms such as, for example, abdominal pain predominantly related to defecation, diarrhea, constipation or alternating diarrhea and constipation, abdominal distention, gas, and excessive mucus in the stool. More than 40% of IBS patients have symptoms so severe that they have to take time off from work, curtail their social life, avoid sexual intercourse, cancel appointments, stop traveling, take medication, and even stay confined to their house for fear of embarrassment. The estimated health care cost of IBS in the United States is $8 billion per year (Talley et al., Gastroenterol., 109:1736-1741 (1995)).
The precise pathophysiology of IBS is not well understood. Nevertheless, there is a heightened sensitivity to visceral pain perception, known as peripheral sensitization. This sensitization involves a reduction in the threshold and an increase in the gain of the transduction processes of primary afferent neurons, attributable to a variety of mediators including monoamines (e.g., catecholamines and indoleamines), substance P, and a variety of cytokines and prostanoids such as E-type prostaglandins (see, e.g., Mayer et al., Gastroenterol., 107:271-293 (1994)). Also implicated in the etiopathology of IBS is intestinal motor dysfunction, which leads to abnormal handling of intraluminal contents and/or gas (see, e.g., Kellow et al., Gastroenterol., 92:1885-1893 (1987); Levitt et al., Ann. Int. Med., 124:422-424 (1996)). Psychological factors may also contribute to IBS symptoms appearing in conjunction with, if not triggered by, disturbances including depression and anxiety (see, e.g., Drossman et al., Gastroenterol. Int, 8:47-90 (1995)).
The causes of IBS are not well understood. The walls of the intestines are lined with layers of muscle that contract and relax as they move food from the stomach through the intestinal tract to the rectum. Normally, these muscles contract and relax in a coordinated rhythm. In IBS patients, these contractions are typically stronger and last longer than normal. As a result, food is forced through the intestines more quickly in some cases causing gas, bloating, and diarrhea. In other cases, the opposite occurs: food passage slows and stools become hard and dry causing constipation.
The precise pathophysiology of IBS remains to be elucidated. While gut dysmotility and altered visceral perception are considered important contributors to symptom pathogenesis (Quigley, Scand. J. Gastroenterol., 38(Suppl. 237):1-8 (2003); Mayer et al., Gastroenterol., 122:2032-2048 (2002)), this condition is now generally viewed as a disorder of the brain-gut axis. Recently, roles for enteric infection and intestinal inflammation have also been proposed. Studies have documented the onset of IBS following bacteriologically confirmed gastroenteritis, while others have provided evidence of low-grade mucosal inflammation (Spiller et al., Gut, 47:804-811 (2000); Dunlop et al., Gastroenterol., 125:1651-1659 (2003); Cumberland et al., Epidemiol. Infect., 130:453-460 (2003)) and immune activation (Gwee et al., Gut, 52:523-526 (2003); Pimentel et al., Am. J. Gastroenterol., 95:3503-3506 (2000)) in IBS. The enteric flora has also been implicated, and a recent study demonstrated the efficacy of the probiotic organism Bifidobacterium in treating the disorder through modulation of immune activity (O'Mahony et al., Gastroenterol., 128:541-551 (2005)).
The hypothalamic-pituitary-adrenal axis (HPA) is the core endocrine stress system in humans (De Wied et al., Front. Neuroendocrinol., 14:251-302 (1993)) and provides an important link between the brain and the gut immune system. Activation of the axis takes place in response to both physical and psychological stressors (Dinan, Br. J. Psychiatry, 164:365-371 (1994)), both of which have been implicated in the pathophysiology of IBS (Cumberland et al., Epidemiol. Infect., 130:453-460 (2003)). Patients with IBS have been reported as having an increased rate of sexual and physical abuse in childhood together with higher rates of stressful life events in adulthood (Gaynes et al., Baillieres Clin. Gastroenterol., 13:437-452 (1999)). Such psychosocial trauma or poor cognitive coping strategy profoundly affects symptom severity, daily functioning, and health outcome.
Although the etiology of IBS is not fully characterized, the medical community has developed a consensus definition and criteria, known as the Rome II criteria, to aid in the diagnosis of IBS based upon patient history. The Rome II criteria requires three months of continuous or recurrent abdominal pain or discomfort over a one-year period that is relieved by defecation and/or associated with a change in stool frequency or consistency as well as two or more of the following: altered stool frequency, altered stool form, altered stool passage, passage of mucus, or bloating and abdominal distention. The absence of any structural or biochemical disorders that could be causing the symptoms is also a necessary condition. As a result, the Rome II criteria can be used only when there is a substantial patient history and is reliable only when there is no abnormal intestinal anatomy or metabolic process that would otherwise explain the symptoms. Similarly, the Rome III criteria recently developed by the medical community can be used only when there is presentation of a specific set of symptoms, a detailed patient history, and a physical examination.
It is well documented that diagnosing a patient as having IBS can be challenging due to the similarity in symptoms between IBS and other diseases or disorders. In fact, because the symptoms of IBS are similar or identical to the symptoms of so many other intestinal illnesses, it can take years before a correct diagnosis is made. For example, patients who have inflammatory bowel disease (IBD), but who exhibit mild signs and symptoms such as bloating, diarrhea, constipation, and abdominal pain, may be difficult to distinguish from patients with IBS. As a result, the similarity in symptoms between IBS and IBD renders rapid and accurate diagnosis difficult. The difficulty in differentially diagnosing IBS and IBD hampers early and effective treatment of these diseases. Unfortunately, rapid and accurate diagnostic methods for definitively distinguishing IBS from other intestinal diseases or disorders presenting with similar symptoms are currently not available. The present invention satisfies this need and provides related advantages as well.